Background Natural or quasi experiments are appealing because they enable the evaluation of events or interventions that are difficult or impossible to manipulate experimentally, which is especially relevant for public health research in which the evaluation of policy and health system reforms is an important focus. There remains ambiguity about their definition and how they differ from randomized controlled experiments and from other observational designs. We conceptualise natural experiments in the context of public health evaluations and align the study design to the Target Trial Framework.
Methods A literature search was conducted, and key methodological papers were used to develop this work. Peer-reviewed papers were supplemented by grey literature.
Results Natural experiment studies (NES) combine features of experiments and non-experiments. They differ from planned experiments, such as randomized controlled trials, in that the assignment of exposure is not controlled by researchers. They differ from other observational designs in that they evaluate the impact of events or processes that resulting from changes in exposure. As a result they are, in theory, less susceptible to bias than other observational study designs. Importantly, causal inference relies heavily on the assumption of ‘as-if randomisation’ of exposure allocation. The target trial framework provides a systematic basis for evaluating this assumption and the other design elements that underpin the causal claims that can be made from NES.
Conclusion Although there will always remain some ambiguity about the strength of causal claims from natural experiment evaluations, there are clear benefits to harnessing these rather than relying purely on observational studies. This includes the fact that NES can be based on routinely available data and that timely evidence of real-world relevance can be generated. Aligning NES to the Target Trial framework will guard against conceptual stretching of these evaluations and ensure that the causal claims about whether public health interventions ‘work’ can inform public health action within a ‘practice-based evidence’ framework.
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